(1) Field of the Invention
This invention relates to flame-retardant liquid photosensitive resin compositions which have pattern-forming properties and can be used as solder resists (or solder masks) in the fabrication of printed circuit boards.
(2) Description of the Prior Art
Conventionally, a solder resist (or solder mask) is widely used in the fabrication of printed circuit boards in order to form a permanent protective coating for printed circuit boards. Such a solder resist is used for the purpose of preventing the formation of a solder bridge during soldering and of ensuring the protection of the conductive parts against corrosion and the retention of their electrical insulation during use. Solder resists used under severe conditions, unlike etching resists, must have the following performance characteristics:
(a) Retention of adhesion during soaking in solder (at 240.degree. to 280.degree. C.). PA1 (b) Permanent retention of adhesion. PA1 (c) Excellent resistance to solvents and chemicals. PA1 (d) Retention of good electrical insulating properties under high-humidity conditions. PA1 (e) Flame retardance. PA1 (a) 10 to 55% by weight of a (meth)acrylate obtained by the reaction of a compound having two or more epoxy groups in the molecule with (meth)acrylic acid and a dibasic acid or its anhydride, the (meth)acrylate having an average acid value of 4 to 150 and a number average molecular weight of 300 to 5,000; PA1 (b) 3 to 35% by weight of a monomer having at least one (meth)acryloyloxy group and containing not less than 20% by weight of bromine; PA1 (c) 10 to 55% by weight of a monomer having two or more (meth)acryloyloxy groups in the molecule, exclusive of components (a) and (b); PA1 (d) 5 to 50% by weight of a monomer having one (meth)acryloyloxy group in the molecule, exclusive of components (a) and (b); PA1 (e) 2 to 35% by weight of at least and PA1 (f) 0.05 to 20% by weight of at least one photopolymerization catalyst selected from the group consisting of photo-initiators and photosensitizers,
In order to meet these requirements, it has been conventional and common practice to form a solder resist by screen printing of a thermosetting ink or a photocurable ink. However, the increasing miniaturization of printed circuits has created a demand for solder resists having large coating thickness and high precision, and it is the existing state of the art that the screen printing method for the formation of a solder resist is no longer satisfactory from the viewpoint of precision and coating thickness.
The formation of solder resists by the development technique has been proposed as a means for responding to this miniaturization or printed circuits. According to the development technique, a pattern is formed by applying a liquid photosensitive resin composition, or laminating a photosensitive film, to a printed circuit board, exposing the coating film to active radiation through, for example, a photomask to cure only desired portions thereof, and then washing the coating film with a developer to remove any uncured portions thereof. This technique makes it possible to form a solder resist pattern having large coating thickness and high precision.
According to the manner in which the coating film to be cured is formed, solder resists for use in the development technique can be classified into three types: dry film type, solvent evaporation type and solventless liquid type (see Japanese Patent Laid-Open Nos. 52703/'77 and 15733/'76). Among them, solder resists of the dry film type have the disadvantage that, in order to cause the solder resist to adhere closely to an uneven surface having a circuit formed thereon, a special process such as heat lamination under reduced pressure is required (see Japanese Patent Laid-Open No. 52703/'77) and that, notwithstanding the use of such a process, perfect adhesion is not always ensured. In contrast, solder resists of the solvent evaporation type can exhibit good adhesion to an uneven surface having a circuit formed thereon. However, they have the disadvantage that, after the application of a liquid photosensitive resin composition, the resulting coating film must be dried in an explosion-proof dryer or similar equipment so as to evaporate the solvent. Accordingly, there is a great need to develop an improved photosensitive resin composition of the solventless liquid type for use as a solder resist.
On the other hand, liquid photosensitive resin compositions can also be classified according to the type of developer used. That is, they include ones using an organic solvent such as 1,1,1-trichloroethane and ones using a dilute aqueous alkaline solution. Since the use of an organic solvent involves problems concerning the pollution of the working environment and the disposal of waste liquid, development with a dilute aqueous alkaline solution is highly desirable. It is known that these problems can be solved by use of compositions containing a carboxyl-modified epoxy (meth)acrylate as an alkali developability imparting agent (see Japanese Patent Laid-Open Nos. 204252/'87, 204253/'87 and 205113/87). However, these compositions do not have a sufficient degree of flame retardancy.